1. Field of the Invention
The present invention relates to a method of assembling an ink jet head unit of a laminated structure in which two ink jet heads each having the same structure as to each other are stuck together, and more particularly to a method of assembling an ink jet head unit which can accurately stick two ink jet heads together.
2. Description of the Related Art
An image recording apparatus for recording images has widely been known. There are various types of image recording apparatuses. For example, an image recording apparatus that uses an ink jet recording system such as an ink jet printer is recently widely used because it is inexpensive. This image recording apparatus records images on a recording medium by ejecting ink thereto. The image recording apparatus includes an ink jet head which is a type of droplet ejection devices for ejecting ink as described above. Hereinafter, a structure of a conventional ink jet head will be described with reference to FIGS. 25 and 26. FIG. 25 is a schematic exploded perspective view of the conventional ink jet head. FIG. 26 is a schematic vertical sectional view of the ink jet head of FIG. 25.
The ink jet head has an ink ejecting portion for ejecting ink. The ink ejecting portion includes a thin flat-plate shaped piezoelectric member 10. The piezoelectric member 10 has one end and the other end. The piezoelectric member 10 has an upper surface 10a extending from the one end to the other end, and a front end face 10b and a rear end face 10c each of which intersects the upper surface. The front end face 10b is positioned at the one end of the piezoelectric member 10, while the rear end face 10c is positioned at the other end of the piezoelectric member. Thus, the front end face 10b and the rear end face 10c are facing in opposite directions.
In the upper surface 10a of the piezoelectric member 10, a plurality of grooves 12 are formed in parallel to each other. The grooves 12 are arranged in a predetermined direction with a predetermined pitch P therebetween. In FIG. 25, each groove 12 extends from the one end of the piezoelectric member 10 to the other end thereof. A boundary portion between adjacent grooves 12 is a side wall for each of the adjacent grooves 12. This side wall extends from one end of the groove to the other end thereof. Each of the plurality of grooves 12 has the same dimensions as to each other. One end of each of the plurality of grooves 12 has an ejection opening 12a. The ejection opening 12a is opened in the front side face 10b. The other end of each of the plurality of grooves 12 has a supply end 12b configured such that its depth becomes gradually shallower as it near to the rear side face 10c, but it does not reach the rear side face 10c. Additionally, each of the plurality of grooves 12 has an upper surface side opening opened in the upper surface 10a of the piezoelectric member 10. This upper surface opening extends from the one end of the groove 12 to the other end thereof in a direction along the groove 12.
An electrode is formed on an inner surface of each of the plurality of grooves 12, the inner surface being constituted by the side walls and a bottom surface of each of the plurality of grooves 12. No reference numeral indicates this electrode to clarify the drawing. On the upper surface 10a, a conductive pattern 14 is formed in a region between the supply end 12b and the rear end face 10c as a conductive means electrically connected to the electrode formed in the groove 12.
The above-described ink jet head has an ink supply means 16 for supplying ink to the piezoelectric member. The ink supply means 16 includes an end flange 16a which covers a region in which the plurality of grooves 12 are opened on the upper surface 10a of the piezoelectric member 10.
The end flange 16a covers the plurality of upper surface side openings of the grooves 12, and is fixed to the upper surface 10a. The end flange 16a has an ink outlet 16c communicated with the supply ends 12b in the upper surface 10a of the piezoelectric member 10.
The ink supply means 16 further includes a small ink container 16e connected to an ink tank (not shown) which is an ink supply source. This small ink container 16e has a connection plug 16d, and is connected to the ink tank through an ink tube (not shown) connected to the connection plug 16d. The small ink container 16e is fixed to a surface of the end flange 16a, the surface facing in a direction opposite to the upper surface 10a, to cover the ink outlet 16c. The small ink container 16e has an ink reservoir 16f into which ink supplied from the ink tube flow. An ink filter 16g is installed in the ink reservoir 16f. 
On the upper surface 10a of the piezoelectric member 10, one end portion of a flexible substrate 18 is fixed to a region in which a plurality of conductive patterns 14 is formed. A plurality of conductive patterns 18a is formed on the flexible substrate 18 to be electrically connected to the plurality of conductive patterns 14. Additionally, on the flexible substrate 18, an integrated circuit (IC) 18b is fixed to selectively send a voltage as a driving signal from an external power source (nod shown) to the conductive patterns 14.
The above-described ink ejecting portion includes a nozzle plate 20 which covers the ejection openings 12a of the plurality of grooves 12 on the front end face 10b of the piezoelectric member 10. The nozzle plate 20 has a plurality of nozzles 20a arranged in positions corresponding to approximate centers of the ejection openings 12a. On the nozzle plate 20, an outer surface opposite to the front end face 10b of the piezoelectric member 10 is treated to repel ink.
Now, an operation of the conventional ink jet head constituted as described above will be described. At first, ink is supplied from the ink tank to the ink jet head. Specifically, ink in the ink tank is pressurized, and the pressurized ink is supplied to the ink jet head. More specifically, the ink is supplied from the ink tank through the ink tube and the connection plug 16d to the ink reservoir 16f of the small ink container 16e. The supplied ink flows through the ink filter 16g and the ink outlet 16c of the end flange 16a into all of the grooves 12 of the piezoelectric member 10. The ink filled in the plurality of grooves 12 may leak through the plurality of nozzles 20a of the nozzle plate 20 to the outside. But, the leaked ink is repelled by the outer surface of the nozzle plate 20 and does not stick thereto.
When the application of the pressure to the ink in the ink tank is released, the pressure of the ink in the plurality of grooves 12 becomes low as compared to the atmospheric pressure. As a result, the ink forms a meniscus in each of the plurality of nozzles 20a of the nozzle plate 20 by surface tension.
The driving voltage is applied to the piezoelectric member 10 while the ink maintains the state of meniscus near the nozzles 20a. More specifically, the driving voltage is applied from a control circuit (not shown) through the flexible substrate 18 to the piezoelectric member 10. Yet more specifically, the IC 18b on the flexible substrate 18, which receives a control signal from the control circuit, selectively applies a driving signal (the driving voltage) through the electrodes in the plurality of grooves 12 to the piezoelectric member 10. For example, the control circuit is a control circuit of a personal computer connected to the image recording apparatus which uses the ink jet head. The groove 12 corresponding to the electrode to which the driving voltage is applied deforms the side walls to narrow the cross sectional area of the groove 12.
The deformation of the groove 12 generates a shock wave that is applied to the ink in the groove 12. By this shock wave, a predetermined amount of ink drop is ejected from the nozzle 20a corresponding to the groove 12 to the outside. The ejected ink drop lands on the recording medium to form a part of an image.
Incidentally, each groove 12 is formed in the upper surface 10a of the piezoelectric member 10 by a rotary cutter blade. Further, the side wall between the adjacent grooves 12 must have sufficient durability because it is deformed as described above. Thus, the piezoelectric member 10 must have a certain thickness. In order to secure the durability, the number of grooves per one inch (25.4 mm) is limited to about 200 at present. The number of grooves formed per one inch is generally 180. In this case, a nozzle density (a density of ejected ink drops) of the ink jet head is 180 dpi.
In recent years, an image recording apparatus which can record an image of little dot granular touch with a higher resolution at a high speed have been demanded. To reduce the granular touch, a size of one ink drop is preferably miniaturized more. However, if the size of ink drop is miniaturized as described above, the nozzle density in the ink jet head must be increased to perform a high-speed recording in a predetermined printing area.
To increase the nozzle density, for example, two ink heads may be stuck together. That is, the higher nozzle density can be achieved by sticking the two ink jet heads to form a single ink jet head unit. To constitute the ink jet head unit, the two ink jet heads are prepared as shown in FIGS. 27 and 28. Then, these ink jet heads are stuck together so that the bottom surfaces of the piezoelectric members 10 of the ink jet heads can be in intimately contact with each other. In this time, the nozzles 20a of one ink jet head are shifted from those of the other ink jet head by a half of one nozzle pitch P, i.e., ½P, along the nozzle arraying direction. That is, one of the ink jet heads and the other are stuck together in such a shifted arrangement.
The ink jet head unit constituted in the aforementioned manner has a nozzle density which is two times of that of the single ink jet head. However, it is difficult to accurately stick the two ink jet heads to each other is difficult.
The present invention is derived from these circumstances, and an object of the present invention is to provide a method of assembling ink jet head unit by which a plurality of ink jet heads can be accurately stuck to each other.